The present invention relates generally to machines for cold forming metal and more particularly to machines for shrink forming metal workpieces.
Shrink forming machines are utilized to decrease the exterior diameter of a tubular workpiece, for example. A shrink former comprises a multiplicity of circumferentially arranged jaws mounted for movement in a radial direction. Located at the radial inward end of each jaw is a die for engaging the exterior surface of the workpiece. Each of the jaws is urged radially inwardly simultaneously, by a hydraulic mechanism, for example. This causes the dies to exert a radially inwardly directed pressure on the exterior of the tubular workpiece, causing metal flow in the workpiece and resulting in a decrease in the exterior diameter of the workpiece. This is usually accompanied by a corresponding increase in the axial dimension of the workpiece to accommodate the displacement of metal arising from the reduction in its exterior diameter. When a tubular workpiece undergoes shrinking, an interior mandrel may be employed to determine the internal diameter of the workpiece, and the external diameter of the workpiece is determined by the extent to which the dies are moved radially inwardly during the shrinking operation.
One type of shrink forming machine is disclosed in Luedi et al. U.S. Pat. No. 3,461,710, and the disclosure therein is incorporated herein by reference.
Shrink forming machines operate on a variety of workpieces having differing initial and final dimensions. A change in the workpiece often requires a change in the dies which perform the shrinking operation on the workpiece, and changing the dies in the shrink forming machine can be a tedious and time-consuming operation. This, in turn, results in substantial shut-down time for the shrink forming machine, which is undesirable.
An important consideration in shrink forming is that the die face must engage the workpiece at a predetermined location and maintain the engagement at that location all during the time the die moves radially inwardly during the shrink forming operation. To accomplish this, it is necessary that the die be carefully guided along a precise radial path during the entirety of its radially inward movement, and any substantial deviation from this path by the die during inward movement is undesirable.
In some conventional shrink forming machines, the die is guided by a key and keyway structure which guides the radially moving machine jaw to which the die is attached, but this guidance occurs at a location which is relatively radially remote from the die face which bears against the workpiece. Thus, a relatively insubstantial deviation at the guidance location is magnified progressively as the radial distance from the guidance location increases in a radially inward direction toward the die face. As a result, a minor clearance between the key and the keyway, or other minor imperfection on the guidance structure causing a relatively insubstantial deviation at the guidance location, can be magnified into a relatively large deviation at the radially remote location where the die face engages the workpiece.
Another problem which occurs in shrink forming arises from the fact that, at the location where the die face contacts the workpiece, very large stresses are developed. The die is made of an expensive alloy material which can withstand this stress, but the shrink forming machine itself, of necessity, is made of a less expensive material which is weaker than the material of which the die is composed. The stresses which are developed at the location where the die contacts the workpiece are transferred from the die through the jaws to the machine. Consequently, provision must be made for the machine to withstand the tremendous stresses which are developed at the die face.